The intervertebral disk is a joint having a function and a structure very complicated functionally and anatomically. It consists of functional structures of anulus fibrosus, vertebral body endplate, and nucleus pulposus. The anulus fibrosus is tough fibrous ring bound to a neighboring vertebral body or a vertebral body endplate. The anulus fibrosus is generally said to have a height of 10 to 15 mm and a thickness of 15 to 20 mm. The nucleus pulposus is placed therein as it is surrounded by the anulus fibrosus. The nucleus pulposus migrates in the anulus fibrosus, when an action such as bending or torsion is applied onto the neighboring vertebral body.
The intervertebral disk may be degraded or damaged by external wound, disease, or aging. In such a case, the nucleus pulposus sticks into the intervertebral foramen, while forming hernia. The nucleus pulposus sticking out pressurizes the spinal cord, causing pain and paralysis of the neighboring structures or pain and paralysis of the lower limbs. Alternatively, it is calcified by aging, becoming less compatible with water and gradually contracted. The nucleus pulposus then cannot retain its favorable interbody distance any more, and cause damage of the upper or lower vertebral body.
Currently, patients with a disease related to the intervertebral disk that plays an important role in the body, such as hernia of intervertebral disk, are treated by interbody fixation by using, for example, an autotransplantation bone, bone cement or synthetic vertebral body spacer after extraction of the affected intervertebral disk. However, these therapies are aimed at fixation of the vertebral body, and the mechanical properties inherent to the intervertebral disk such as flexibility and load-buffering action are neglected. As a result, such a therapy exerts an adverse effect on the neighboring upper and lower vertebral bodies, causing secondary damages of the intervertebral disks.
On the other hand, the hernia of intervertebral disk is known to be induced for example by aging of the nucleus pulposus. In the case of a relatively milder disorder, a therapy of replacing only the aged nucleus pulposus is more desirable than the vertebral body fusion. Thus, it is preferable to perform local or entire replacement by using a synthetic prosthesis having a function to assure sufficient mobility of the intervertebral disk, while mimicking the natural physiological function of the intervertebral disk and preserving the normal interbody space. they also had some problems. These prostheses are large implants demanding partial removal of the neighboring upper and lower vertebral bodies and entire replacement of the intervertebral disk and the neighboring structures, and thus, had a possibility of imposing a very high load on the patient during operation. In addition, such a device was higher in rigidity and thus, undesirable from the point of material mechanics.
Accordingly, these prostheses smaller in size were developed. Such a smaller prosthesis is intended to be used for replacement only of the nucleus pulposus in the intervertebral disk. For example Bao Qi-Bin et al. studied replacement of the body nucleus pulposus with a hydrogel (see Patent Documents 1, 2, and 3). Alternatively, Charles D. Ray et al. also prepared a prosthesis in combination of polyethylene and a hydrogel aimed at replacement of the body nucleus pulposus (see Patent Documents 4 and 5). PVA hydrogels, which have favorable repeated load resistance and shock-relaxing property even under repeated pump action of absorbing and releasing structural water into and out of the hydrogel, are favorable as the materials for synthetic nucleus pulposus prosthesis.
Such a synthetic nucleus pulposus prosthesis is inserted into the space between the anulus fibrosus and a vertebral body cartilage called vertebral body endplate at the affected intervertebral disk site. The inserted synthetic nucleus pulposus prosthesis in the body anulus fibrosus is considered to move vigorously vertically and horizontally occasionally under a load of several times larger than the body weight.
The synthetic nucleus pulposus prosthesis may possibly be separated from the anulus fibrosus under a load of varying magnitude. If it is separated, the intervertebral disk that had the separated synthetic nucleus pulposus prosthesis may not have sufficient load resistance and damage the intervertebral disk and the neighboring upper and lower vertebral bodies. In addition, the implant itself has a possibility of damaging the neighboring structures and nerves. Thus, it is quite important to prevent separation of the synthetic nucleus pulposus prosthesis after operation.
In replacement of the body nucleus pulposus with a synthetic nucleus pulposus prosthesis, it was necessary to form an opening in the anulus fibrosus and an insertion route thereto. If a synthetic nucleus pulposus prosthesis is separated, it would be separated from the opening and the insertion route thus formed. Reduction in size of the opening and the insertion route leads to fewer frequency of the separation of synthetic nucleus pulposus prosthesis. In addition, it also leads to reduction of the damage on anulus fibrosus and thus, to decrease in the possibility of complications induced. Further, if the replaced synthetic nucleus pulposus prosthesis, after insertion, happens to become larger than the opening and the insertion route formed in the anulus fibrosus by some denaturation or deformation, it would not be separated in principle from the opening or the insertion route.
For that reason, Bao et al. disclosed a prosthesis prepared with a hydrogel in the study described above, which was inserted into the anulus fibrosus in the dehydrated state and expanded therein by hydration. The study by Bao et al. and other similar studies depended only on hydration of the hydrogel and demanded a very high water content of about 55 to 99% for desirable expansion. As a result, the resulting prosthesis is lower in load resistance, possibly causing recurrence of the hernia condition by deformation under application of high load. In addition, reconstruction of the structure is dependent on the condition of the surrounding anulus fibrosus. Thus, incomplete condition of the anulus fibrosus may result in separation of the prosthesis without functioning as the intervertebral disk. In addition, a prosthesis with insufficient load resistance demands an instrument for fixing the vertebral body for compensation of low load resistance, which may lead to increase of the load on the patient. Swelling of the hydrogel is restricted only by the body anulus fibrosus. The essentially uncontrolled swelling may raise a concern about recurrence of the hernia condition. In addition, it may prevent healing of, or even worsen, the anulus fibrosus opening that was cut open for insertion. Actually in an animal test by using baboons, there were 6 cases of separation in the 20 cases tested. Further in some of them, the separation occurred from the sites other than the inserted anulus fibrosus opening site, indicating that the load resistance was insufficient (see Non-patent Literature 1).
Ray et al. also studied a prosthesis of hydrogel system, which was also inserted into the anulus fibrosus in the dehydrated state. In the prosthesis, expansion of the hydrogel was restricted as it is covered with a polymeric woven-fabric jacket such as of polyethylene on the periphery. However, because the polymeric woven-fabric jacket used was relatively rigid and almost resistant to shrinkage, swelling, denaturation, or deformation, the prosthesis had a problem that it was larger also in the dry state. Thus, improvement was made to insert two prostheses that were significantly smaller in size than the anulus fibrosus lumen. In the method of inserting two prostheses, a small incision opening is formed in the anulus fibrosus and the prostheses are inserted one by one. This prosthesis-inserting method is advantageous in that the incision opening of the anulus fibrosus is smaller, but the shape of the substantially smaller prosthesis and that of the anulus-fibrosus incision opening are almost the same, and under application of a mechanical force such as bending or torsion, the individual prostheses may be separated out of the anulus fibrosus one by one. To solve the problems above, the two prostheses are bound to each other with a string. It solved some of the problems associated with separation. However, because the core hydrogel has a high water content, the prosthesis also raises a concern about its low load resistance. During movement of the prosthesis in anulus fibrosus, the relatively rigid polymeric woven-fabric jacket may damage the upper and lower vertebral body cartilages significantly and thus, such a prosthesis is not favorable as a prosthesis for replacement of nucleus pulposus surrounded by the endplate cartilage and the anulus fibrosus. Further, the two prostheses inserted into the anulus fibrosus raises concerns about abrasion between them, generation of unexpected abrasion powder, and breakage of the polymer woven fabric jacket.
Currently in the rapidly graying society, diseases caused by the intervertebral disk denatured or damaged by aging, external wound, or disease are significant threat to patients. Thus, proper recovery of the function of the damaged intervertebral disk is important.
However, because the synthetic nucleus pulposus prosthesis described above has a practical water content of approximately 55 to 99 wt %, it raised a concern about low load resistance and recurrence of the hernia condition by deformation under application of high load. Further, reconstruction of the structure is dependent on the condition of the surrounding anulus fibrosus. Thus, incomplete condition of the anulus fibrosus may result in separation of the prosthesis from anulus fibrosus without functioning as the intervertebral disk and possible occurrence of damage caused thereby. In addition, a prosthesis with insufficient load resistance demands an instrument for fixing the vertebral body for compensation of low load resistance, which may lead to increase of the load on the patient. Further, abrasion with the neighboring vertebral body cartilage endplates may also cause damage of the peripheral structures.
Bao et al. reported a method of inserting at least one hydrophilic xerogel rod into the cavity of anulus fibrosus having no the natural nucleus and completely filling the cavity by expansion of the hydrogel by water absorption (Patent Document 6). However, it was not possible to control the shape of the hydrophilic xerogel rod in anulus fibrosus, and disadvantageously, the densely filled site raised a concern about recurrence of hernia, while the coarsely filled site raised a concern about insufficient load resistance. It also caused a concern about generation of unexpected abrasion powder by abrasion among xerogel rods inserted and bent in the anulus fibrosus.    Patent Document 1: U.S. Pat. No. 5,047,055    Patent Document 2: U.S. Pat. No. 5,192,326    Patent Document 3: U.S. Pat. No. 5,976,186    Patent Document 4: U.S. Pat. No. 5,824,903    Patent Document 5: U.S. Pat. No. 6,132,465    Patent Document 6: Japanese Unexamined Patent Publication No. 8-98851    Non-patent Literature 1: “Preclinical Evaluation of a Poly(Vinyl Alcohol) Hydrogel Implant as a Replacement for the Nucleus Pulposus”, SPINE, VOL. 29, NO. 5, 2004